The present invention relates to data transmission systems and more particularly, concerns an automatic filter for an Asymmetric Digital Subscriber Line system to mitigate narrowband interference.
Asymmetric Digital Subscriber Line (xe2x80x9cADSLxe2x80x9d) is a technology which allows for high-speed data communication simultaneously with voice traffic over twisted-pair phone lines. Twisted pairs, which can be used to connect a central telephone system to a subscriber""s telephone system can support bandwidths up to 2 MHz through the use of digital signal processing (xe2x80x9cDSPxe2x80x9d) technology. Full rate ADSL transmission systems based on American National Standard Institute (xe2x80x9cANSIxe2x80x9d) T1.413 and International Telecommunication Union (xe2x80x9cITUxe2x80x9d) G.992.1 operate in a frequency range from approximately 25 kHz to 1.1 MHz. Transmission rates in accordance these ADSL standards are intended to facilitate the transmission of information at rates of up to 8 million bits per second over twisted-pair phone lines.
The standardized system defines the use of a discreet multitone (xe2x80x9cDMTxe2x80x9d) modulation method that transmits information in 256 xe2x80x9ctonesxe2x80x9d or sub-bands that are each 4.3125 kHz wide in the forward or downstream direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office to a remote location that may be an end user such as a residence or business.
The ADSL standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 kilobits per second. The reverse signal corresponds to transmissions in an upstream direction, i.e. from the remote location to the central office. The term xe2x80x9cADSLxe2x80x9d results from the xe2x80x9casymmetricxe2x80x9d data transmission rates in the upstream and downstream direction.
In nearly all ADSL implementations, twisted-pair phone lines are used as at least a part of the transmission medium that connects the central office to end users. The twisting of the twisted-pair provides some protection against external radio frequency interference (xe2x80x9cRFIxe2x80x9d). However, the twisted-pair phone lines exhibit antennae-like properties at high frequencies. As a result, RFI becomes a significant contributor to noise within the transmission system, particularly as distance and frequency increases. This can become particularly problematic in very high speed DSL (xe2x80x9cVDSLxe2x80x9d) systems.
The information capacity of each DMT sub-band depends on many factors including RFI ingress. For low amplitude RFI, approximately only 2 DMT sub-bands are affected. However, field experience with ADSL has shown that in the proximity of high power radio transmitters, RFI can affect a large number of DMT sub-bands because of the limitations of sub-band filters. In addition, in some cases, a high level of RFI can exceed the dynamic range of the analog-to-digital converter in the ADSL receiver resulting in a total failure of the ADSL system. Standard ADSL systems which operate in a frequency range from approximately 25 kHz to approximately 1.1 MHz are particularly susceptible to RFI since AM radio broadcasts which comprise of principal source of RFI, operate at frequencies above 535 kHz.
A balanced digital subscriber loop comprising a twisted wire pair carries both differential and common mode currents. The differential currents carry the information signal, and the common mode currents are typically associated with noise sources. One form of common mode noise is RFI.
Various techniques are known for reducing interference or noise in a communication channel. For example, U.S. Pat. No. 6,052,420 entitled xe2x80x9cAdaptive Multiple Sub-Band Common-Mode RFI Suppressionxe2x80x9d discloses a technique for canceling common mode noise on a two-conductor communications channel. This technique involves a device for extracting from the channel a corresponding common mode signal and a noise estimation unit for deriving from the common mode signal a noise estimate signal representing a common mode noise level in a selected frequency band which is narrower than the operating band width of the differential signal. The noise estimation unit adjusts the amplitude of the noise estimate to correspond to the residual noise in the differential mode signal and subtracts it from the differential mode signal to produce a noise-suppressed output signal. The noise detection and control units scans the operating band to identify the frequency bands having the highest noise level and uses the noise estimation unit to suppress the noise in those bands. This technique is not entirely satisfactory because it relies upon the common mode signal to generate the noise suppression signal for the differential mode signal. Moreover, this system requires a high correlation between the common mode and differential mode noise signals, and is also highly dependent on a high quality ground connection. In practice, a high quality ground connection may be difficult to achieve and the common-to-differential mode correlation may be frequency dependent.
Accordingly, an object of the present invention is to eliminate or mitigate the disadvantages of the foregoing known technique and provide an automatic filter for suppressing narrowband noise such as RFI in an ADSL system.